SMAD4 and colorectal cancer: Rarely, mutations or deletions of the SMAD4 gene occur in other cancers, such as colorectal cancer [150]; head and neck cancer, in which it is associated with metastasis and in cancers of the appendix, including mucinous neoplasms of the appendix, with possible loss of function [7]; appendiceal adenocarcinomas [15,16,30,33], in which their frequency exceeds 10%; and low-grade appendiceal mucinous neoplasms, appendiceal goblet cell adenocarcinomas, and well-differentiated neuroendocrine tumors of the appendix [37].